Since the original manufacture of dry cells or batteries, a user cannot tell from looking at them whether they have little useful life left or whether they are fresh. A completely discharged battery and a new one present the same physical appearance. To determine the usefulness of a battery, one would think that one could simply put the battery into a device and operate the device. Very few devices, however, work with only one battery and if a new battery and a discharged battery, or a nearly discharged battery, are put into a device such as a flashlight, the flashlight will glow very dimly, if at all. The conclusion, then, is that the batteries are all defective and should be replaced when, in fact, one of the batteries might be a fresh battery.
In order to test the cells, one can use relatively expensive testing equipment such as a voltmeter or an ammeter but this is an inordinate expense in comparison to the cost of a new set of batteries.
There are other proposals for testing batteries. For example, U.S. Pat. No. 4,723,656 issued Feb. 9, 1988, to Kiernan et al. discloses a blister-type package for new batteries with a battery condition indicator built into the blister portion of the package. The blister can be deformed to place the tester across the terminals of a battery to be tested. A thermochromic liquid crystal material is employed in the tester in combination with a wedge-shaped resistive element. As the current flows through the resistive element, a thermal front will move away from the narrowest portion of the wedge to an extent depending on the capacity of the battery. The effect of the thermal front moving can be seen in the liquid crystal layer in thermal contact with the resistive element.
Several patents have been issued to Robert Parker, for example, U.S. Pat. Nos. 4,702,563; 4,702,564; 4,726,661 and 4,737,020 relating to the use of a heat generating pattern on a flexible substrate in combination with a thermochromic liquid crystal layer to determine the state-of-charge of a battery. Attempts have even been made to view the contents of the cell while in use and to detect a color change in the working components of the cell to determine the state of the cell, for example U.S. Pat. No. 4,497,881 issued Feb. 5, 1985, to Bertolino. U.S. Pat. No. 3,667,039 issued to Garfein et al. on May 30, 1972, discloses the use of liquid crystal materials in a closed cell having either a shaped electrode or a shaped cell in order to provide a field gradient across the liquid crystal material. The cell is then connected across the terminals of a battery and the state of the battery is determined by noting the location of a visible gradient in the liquid crystal material and reading from an accompanying scale the status of the battery.
U.S. Pat. No. 4,835,476 issued to Kurosawa discloses heat sensitive dye systems using organic materials such as crystal violet lactone in combination with other reactive materials to form compositions which decolorize on application of the heat produced by current flowing through a resistive element connected between the terminals of a battery. When the coating material decolorizes, a scale beneath the coating becomes visible indicating the status of the battery.
It can be seen from the above discussion that most attempts in the past have used thermochromic materials, that is, materials which undergo a change in color or color intensity on the application of heat. Since heat is used to change the color of the material, it is important that the heat not be drawn away from the measuring device. It has, therefore, not been practical to employ such devices attached directly to the dry cell or battery to be tested. The large thermal mass presented by the battery acts as a heat sink drawing the heat away causing the measuring device to read inaccurately. A battery condition tester has long been desired for incorporation on the cell or battery rather than on the battery package. Such an on-the-cell tester would be more convenient for the customer since it would be an integral part of the cell or battery and would not be discarded with the original battery package.